1. Field of the Invention
The present invention relates to a diversity circuit and a frame phase (or sampling timing) estimation circuit which uses the diversity circuit, in digital radio communication.
2. Prior Art
FIG. 1 is a block diagram illustrating a prior art diversity circuit such as the one disclosed in the literature titled "A New Post-detection Selection Diversity with MLSE (Maximum Likelihood Sequence Estimation) Equalization" (Autumnal National Session of the Institute of Electronics, Information and Communication Engineers, 1989, B-502, pp. 2-172). In FIG. 1, numeral 10 designates a comparison circuit adapted to compare a power ratio, numeral 11 a selection circuit adapted to output the optimum received signal and an estimated value of the channel characteristics such as an estimated channel impulse response (CIR) respectively to output terminal 15 for the selected received signal and output terminal 16 for the selected estimated CIR value, numerals 31, 32 respectively a first CIR estimation circuit and a second CIR estimation circuit adapted to estimate CIR on the basis of the received signals input to the input terminals 12, 13 and a transmission pattern which is known, and numerals 33, 34 respectively a first power ratio computing circuit and a second power ratio computing circuit adapted to obtain the power ratio of the estimated CIR.
Operation of the diversity circuit will next be explained by referring to a flow chart shown in FIG. 2. The signals which have been received by antennas (not shown) installed at different locations spaced from each other are detected by respective detection circuits (not shown) and then input to the input terminals 12, 13 as the received signals. The first CIR estimation circuit 31 and the second CIR estimation circuit 32 receive the respective received signals and compute estimated CIR values for the respective input systems and output the computed values (Step ST21). Then the first power ratio computing circuit 33 and the second power ratio computing circuit 34 are adapted to receive the respective estimated CIR values and compute the ratio of the power component to be used in the maximum likelihood sequence estimation (MLSE) section at the later stage (not shown) (i.e., the power within the processing range) relative to the power out of the range of MLSE equalization (i.e., the power out of the processing range) (Step ST22). The comparison circuit 10 is adapted to receive the respective power ratios and output a selected signal designating the received signal which corresponds to the largest power ratio or the received signal which is supposed to have the largest equivalent S/N ratio. The selection circuit 11 is then adapted to output the received signal and the estimated CIR value corresponding to the selection signal.
FIG. 3 is an explanatory diagram showing an example of the relationship between the estimated CIR and the actual CIR. In FIG. 3, the solid line designates the estimated CIR while the dotted line designates the actual CIR. In the case as illustrated by FIG. 3A, the power ratios of the CIR power within the processing range relative to the CIR power out of the processing range are computed and the power ratio designating that the power is most concentrated in the processing range is selected. Here consideration is made in respect of the case that the estimated CIR values for the received signals input to the two input terminals 12, 13 have exactly the same values but only the noise power respectively corresponding to these received signals are different from one another. In this case, the received signal corresponding to the smaller noise power should be selected. However, according to the comparison method of power ratios in the diversity circuit as above explained, the probability of such a selection being made is 0.5. Another consideration is also made in the case in which the estimated CIR values for the received signals are exactly the same but the relationship between the estimated CIR values and the actual CIR values are respectively as illustrated by FIGS. 3A and 3B. In this case, the received signal corresponding to the smaller error in CIR estimation shown in FIG. 3A should be selected, but the probability of such a selection being made 0.5.
Since the prior art diversity circuit does not consider the noise power and CIR estimation error upon selection of the received signal, proper selection is often not made when the noise power and CIR estimation error are considerable.